ttcn3-hnbgw-test: reduce X31 to 5s
Set X31 to 5s as expected by the testsuite to fix currently failing:
- TC_ranap_cs_mo_disconnect
- TC_ranap_ps_mo_disconnect
It was recently increased to 15s in the related patch.
Related: osmo-hnbgw I24225cfc0addf326c239ec658a27b93b83a3e751
Change-Id: I223d38e9ec2ca0f9f2ce2ac5311932789f328c9a
